Any kind of artifical flavor is going to affect insulin response/blood sugar...so if your purpose of a fast is to not have an insulin response...then yes it would technically break the fast. And how something with "maltodextrin" is sugar free?? Ummmm....someone is taking liberties with the "serving size"...as if it is under 0.5grams then the FDA says you can list it as 0grams...so yes anything can be sugar free if you get the "serving size" small enough.

Comes down to if you are eating fruits and vegetables, and avoiding processed foods and other bad substitutions (like sausage, hot dogs, etc...), you don't need extra fiber as you will get enough. That and avoid bran like the plague, it's an anti-nutrient. If you want to do a fiber load to clear out the system for a week....feel free as a little gut housecleaning is always good (do water, flaxseeds grounded, some acv or probiotics...your own personal Colon Blow...personally wanting to try some Bentonite clay as I hear that pulls all sorts of "stuff" out). But there is no real proof that high fiber actually reduces any kind of prostate cancer (which is why your Dr recommends it). Eat right foods in the first place, get enzymes to break it all down and you don't need all the other things as it is digested properly and the body will get rid of the waste....funny how it all comes full circle...healthy meat, vegetables, some fruit, healthy fats.....all you need.

Grind up some flaxseeds (I've used up to four Tbsp./day at some points) in a blender or devoted coffee grinder. Mix with a relatively small amount of water and pound it down. Fiber, O-3 fatty acids, and lignans without all the artificial (and wholly unnecessary) junk additives.

Don't use the pre-ground stuff. One has no idea how long it has been ground or how rancid the oil has gone.

John, any calories will 'techinically' break a fast, because they are providing energy to your body. The benefits of fasting are all about persuading your body to look inward for its energy needs, not outward to incoming dietary calories. That said, just how much small calorie intake reduces the benefits of total calorie fasting is anyone's guess. Suffice to say, the fewer calories you consume during a 'fast' the better.

It's also worth mentioning that even fibre itself provides some calories because lower gut bacteria eat it and produce various fatty acids that the human body then uses as an energy substrate. In other words fibre provides a small amount of extra fat (about 2 cals/gm)

I'm also a bit curious why you don't just consume the fibre when you aren't fasting? Most intermittent fasters eat at least every 48 hrs after all, which is plenty regular to keep you regular if you consume fibre as part of those meals.

Fasting is a adjustable term....as you have:
- water fasting
- juice fasting
and any other term you want to "fast" or a break from....

Juice is more 3+ days where you need energy....for IF terms since we are doing shorter fasts (18hr) then water only would be the advised way. It's a very short break for the body to use it's internal energy and resources to heal and rebuild...so adding in any calories in that short period of time will take away from the whole fasting experience. I also sip on ACV in water when I think of it for fasting times....and also an Americano or tea. There's benefit to any version of fasting...but the most bang for your buck will obviously be water only (with added lemon or acv).